Unfortunately, as our 'DD type SP', it's only great if you're fighting something with a great buff/SP to steal, eg a war, mnk or drk (possibly sam) mob that can use 2 hour.
Aside from DDing, it's great to mitigate damage either by stealling chainspell/Manafont form a mob like AV, or by stealing Perfect Dodge/Invincible to reduce your own damage taken or just speed up a fight to allow you to kill it faster.

As for using multiple THF in legion....

A WAR with Mighty Strikes (extendable through relic+2 aug) and perma-double attack and Ragnalution or Ukko's spam will still do way more damage than a THF that stole 25 secs of Mighty Strikes trying to spam Exenterator/Rudra's. Oh and lolperfect dodge.
No, a THF would be there for the same reason as a SMN would be. To mitigate damage taken by the DDs. And you already have SMN there for that. And remember that when this ability comes out, SMN also gets the ability to back-to-back spam BPs straight after they PD'd the DD party.